#include <bits/stdc++.h>
using namespace std;

constexpr int inf = 1001001001;

int dp[5005][10005];

int main() {
    int N;
    cin >> N;
    for(int i = 0; i <= N; i++) {
        for(int j = 0; j <= N; j++) {
            dp[i][j] = inf;
        }
    }
    int sum = 0;
    vector<int>A(N);
    pair<int,int>x;
    vector<pair<int,int>>tmp;
    for(int i = 0; i < N; i++) {
        cin >> A[i];
        if(A[i] == 2) {
            tmp.push_back(x);
            x = {0,0};
        }
        else if(A[i] == 0) {
            x.first++;
            sum++;
        }
        else {
            x.second++;
        }
    }
    tmp.push_back(x);
    dp[0][0] = 0;
    for(int i = 0; i < tmp.size(); i++) {
        for(int j = 0; j <= N; j++) {
            dp[i+1][j] = min(dp[i+1][j],dp[i][j]+tmp[i].first);
            dp[i+1][j+tmp[i].first+tmp[i].second] = min(dp[i+1][j+tmp[i].first+tmp[i].second],dp[i][j]+tmp[i].second);
        }
    }
    if(dp[tmp.size()][sum] == inf) {
        cout << -1 << endl;
    }
    else {
        cout << dp[tmp.size()][sum]/2 << endl;
    }
}